1. Field of the Invention
The invention disclosed herein relates to geophysical assessments using nuclear magnetic resonance techniques and, in particular, to techniques for discerning water from hydrocarbon fluids.
2. Description of the Related Art
Many petroleum reservoirs around the world contain salty formation water. Formation water can be of meteoric or connate origin, or from a mixture of both. Although seawater contains salt of about 30 kppm, the average petroleum reservoir having connate brine contains about 80 kppm of salt. Many petroleum reservoirs that are saturated with salt, such as those common to the major oil-producing or gas-producing fields in the Middle East, contain salt as high as 250 kppm, (i.e., the salt may represent up to about 25% of the weight of the brine). Connate brine contains mainly NaCl salt.
Current NMR wireline and logging techniques only use measurements for protons for a good reason. The proton has a natural abundance of nearly 100%. In addition, the proton has the strongest per-sample signal intensity, as determined by f03I(I+1), where f0 is the NMR frequency and I is the spin quantum number. Because both the hydrocarbon phase and water (brine) phase contain protons, properties of each phase can not be determined from the proton signal intensity alone. Accordingly, the capability of traditional proton NMR techniques for discerning hydrocarbon from salty water downhole has its limitations.
Further, since brine is a conductive liquid, its presence in the formation has a negative effect on the signal quality of many other logging instruments, considering those instruments that are based on the transmitting and receiving of electrical power. Such instruments may include, but are not limited to, nuclear magnetic resonance (NMR) logging instruments and galvanic and inductive logging devices. Poor signal to noise ratio in a salty environment makes it even more difficult to discern hydrocarbon from water.
The current arts of NMR logging techniques discern hydrocarbon from brine based on their contrast in relaxation times (i.e., the longitudinal relaxation time T1 and the transverse relaxation time T2) and the fluid diffusivities, D. Although these techniques have been used successfully downhole, the applicability of these approaches relies heavily on the robustness of these contrasts and the adequate signal-to-noise ratio (SNR). For light hydrocarbons such as volatile and very low-viscosity oils, the water and oil diffusivity contrast is very small; it would be very difficult to separate oil and water from NMR responses, especially in the rocks with large pores and in vuggy carbonates.
On the other hand, determination of formation water resistivity is an important step for induction or resistivity-based saturation estimates. Although in many mature reservoirs the Rw value is well known, for exploration wells, it is important to be able to determine formation resistivity or the equivalent of formation water salinity.
The poor signal-to-noise ratio (SNR) and the possibility of mixing 23Na signals from borehole with signals from protons (i.e., hydrogen) in formation fluids have been the main reason for not considering 23Na NMR logging measurements using the current NMR logging instrument configurations.
Therefore, what are needed are techniques for discerning a presence of hydrocarbon materials from salty water downhole. Preferably, the techniques may be implemented using existing technologies, such as instruments using nuclear magnetic resonance (NMR).